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A sluggish Porsche, it can't be!
Start with the basics. The first thing is to check the compression. See if you have a solid engine.
The noise could be a number of things, but the compression test may help to determine what is causing it. The noise and sluggishness might be because of the cam timing off.
Find a good tech. This car is a work of art and mechanical perfection. It needs the right kind of person to work on it. A Porsche club in your area will provide the names of the best techs for this car.
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It might still need an engine overhaul. But you have to start someplace.
If for example, an exhaust valve was burnt in number 5 cylinder, and the previous owner drove the car for a long time, it will eventually destroy the rod bearing. But the compression test is the best place to start. If all the cylinders have low compression, it could have jumped time (cam timing) causing the lack of power and the noise.
If you do a compression test and everything is fine, then something else is causing the lack of power and there may be no deep engine problems. When the compression test is done, if a cylinder is low, a compression leak down test can be performed. If you find that you have 35% leakdown past the piston, then it's time for a rebuild.
There are a lot of different that could be wrong, but the logical place to start is always a compression test.